Present invention relates to systems and method for reconstructing a femoral portion of a knee in preparation for a total knee replacement. More particularly, the invention relates to systems and methods for orienting a cutting block used to resect portions of the femur, and more specifically in such as way as to balance medial and lateral collateral ligament tension.
The typical total knee arthroplasty includes three components, as depicted in FIG. 1. In particular, the knee prosthesis 10 includes a femoral component 12, a spacer 13, and a tibial component 14. The design of the components of the knee prosthesis 10 is complicated by the fact that the natural knee does not operate like a hinge moving about a single axis. The knee exhibits dynamic external rotation of the tibia relative to the femur as the knee moves from its flexed to its fully extended position. This automatic rotation of the tibia occurs in the opposite direction when the knee is flexed from its fully extended position to produce an internal rotation of the tibia relative to the femur.
Thus, the natural knee exhibits a rotary laxity that allows the tibia to rotate through a limited arc of both internal and external motion at every degree of knee flexion. While the configuration of the components 12-14 themselves is important to achieve proper knee motion, the specific placement of these components is also critical. Most particularly, the proximal/distal and anterior/posterior positions of the femoral component 12 are critical for duplicating the kinematics of the knee. Much literature and clinical study has been devoted to determining the proper orientation of the prosthesis components, and particularly the femoral component 12.
As is known in the field of total knee arthroplasty, and as can be discerned from FIG. 1, the femoral component 12 exhibits a three-dimensional saddle configuration. The interior of the component is configured to mate with a prepared end of the femur. For the typical femoral component, the orthopaedic surgeon must make five cuts at the proximal end of the femur, as illustrated in FIG. 2. In a typical procedure, a distal cut 1 is made across the distal end of the femur F. The anterior-to-posterior (A/P) placement of the femoral component is determined by the anterior and posterior cuts 2 and 3, respectively. Once those cuts are complete, angled chamfer cuts 4 and 5 are typically made to help secure the femoral component 12 solidly in place. In order to effectively and accurately make these resections, orthopaedic surgeons will typically use a cutting block or cutting guide that is temporarily affected to the distal end of the femur F.
One important factor in determining the proper position of the femoral component, and therefore the proper location for the various cuts 1-5, is the tension in the adjacent soft tissues, particularly the collateral ligaments. Proper tension in the collateral ligaments prevents sideways toggle of the leg. In addition, the collateral ligaments help to limit anterior/posterior travel of the femur relative to the tibia. The spacer 13 (FIG. 1) is an important component for maintaining the proper tension in the collateral ligament. With respect to a prosthetic knee, such as the prosthesis 10, the spacer 13 is necessary to account for the gap that occurs between the distal end of the femur F and the proximal end of the tibia, after portions of both bone ends have been resected.
Knee instruments have evolved that provide means for mechanically distracting the knee joint to exert the proper tension on the ligaments during the knee arthroplasty procedure. One such instrument is illustrated in FIGS. 3-5. In particular, a distraction device 20 can include a lower member 22 that is adapted to contact the tibia plateau P (FIG. 4) of the tibia T. The distraction device also includes an upper member 24 that contacts the distal end of the femur F, preferably after the first distal cut (cut 1 in FIG. 2) has been performed. With this type of distraction device, the upper member 24 includes a medial and a distal member 24a and 24b, respectively, as depicted in FIG. 5. A pair of parallel adjusting members 26 adjust the position of the two upper members 24a, b relative to the lower member 22. Preferably, the adjusting members 26 operate independently so that the medial upper member 24a can be adjusted differently from the distal member 24b. This independent adjustability of the medial and distal members allows the surgeon to properly orient the mechanical axis of the knee joint relative to the leg bones. Each adjusting member 26 can include an indicator 27 which provides an indication of the joint space, and ultimately the amount of tension applied to the ligaments. The distraction device 20 can be of a variety of types. For the purposes of illustrating the preferred embodiment of the present invention, a distraction device such as the device described in U.S. Pat. No. 4,501,266, issued Feb. 26, 1985, can be implemented. The details of this devise set forth in the ""266 Patent are incorporated herein by reference.
In a typical knee arthroplasty procedure, the ligament tension is evaluated with the knee joint in its extended positionxe2x80x94i.e. with the femur F and tibia T in alignment. In this position, the prepared surfaces of the femur and tibia are distracted apart by an extension gap G. The ligament tension can be corrected by releasing soft tissue attachments if a medial-lateral imbalance exists, thereby producing a symmetric or parallel extension gap. With this approach, the remainder of the arthroplasty procedure is geared toward maintaining this extension gap.
However, this approach does not account for proper tensioning of the knee when it is in flexionxe2x80x94i.e., when the knee joint is bent. It is with this type of movement that the A/P position of the femoral component 12 is most critical. If the component is offset incorrectly in the A/P direction, the flexion gap will not be correct and will not generate the proper amount of tension in the collateral ligaments. Thus, while the xe2x80x9cextension gap firstxe2x80x9d approach generally assures a proper extension gap, this same approach does not translate to insuring a proper flexion gap. If the flexion gap is incorrect, the ligaments may be too loose or too tight, or the range of motion of the knee may be limited. In addition, poor tensioning in flexion can led to improper femoral rotation, condylar lift-off, accelerated wear of the prosthesis components, and even spin-out or dislocation of the femoral-tibial articulation. It is therefore important to derive a proper A/P position for the femoral component 12.
Some procedures exist which begin with evaluating the flexion gap and then utilizing this flexion gap as the benchmark for the sizing and positioning of the knee prosthesis components. An approach of this type is shown in U.S. Pat. No. 6,056,756, issued May 2, 2000. While the system and method in ""756 Patent correlates to the flexion gap, the A/P placement of the femoral component is made with reference to bony land marks on the femur, and not with reference to the flexion gap itself. Moreover, the approach in ""756 Patent makes no account for a proper extension gap, except to the extent that sizing of the prosthesis components relative to the flexion gap achieves similar results when the knee is in extension.
Consequently, there is a need for an instrument and technique that more accurately locates the femoral component on the prepared distal end of the femur. This need is particularly acute with respect to the A/P placement of the femoral component, which is ultimately a function of the resection cuts made in the distal end of the femur (see FIG. 2). Thus, the need extends to providing an instrument and method for accurately orienting and positioning the cuts that need to be made in the femur to achieve a properly sized femoral component.
To address these needs, the present invention contemplates a flexion gap instrument that is configured to mate with a ligament tensioning tool, such as known distraction devices and pivoting knee balancers. The instrument allows A/P positioning of a cutting block or other knee arthroplasty device to produce a proper flexion gap in the reconstructed knee.
In one embodiment of the invention, the flexion gap instrument includes a base that is configured to sit within the ligament tensioning device, and a vertically extending upstanding portion that is configured to mate with a cutting block or other knee arthroplasty instrument that must be positioned relative to the distal end of the femur. The upstanding portion in one embodiment is an I-beam upstanding portion formed by a back plate and a forward measurement plate. The measurement plate can include a scale or other marker indicating a flexion gap distance which can be used to orient and align the cutting block in its proper position and orientation relative to the distal end of the femur. In a preferred embodiment, the vertically extending upstanding portion is substantially perpendicular to the base. The upstanding portion is arranged so that the supported cutting block is oriented to the cut tibia while the ligaments are tensed to create a symmetric (balanced medial-lateral) flexion gap.
In certain embodiments, a leaf spring can be incorporated into the I-beam to help support the cutting block and provide means for temporarily positioning the cutting block on the flexion gap instrument without the need for additional or supplemental locks. In other embodiments, the vertical extending upstanding portion is in the form of a T-shape for slidable positioning within a correspondingly shaped channel in a cutting block. An alternative feature of the invention contemplates placing the measurement plate at a non-perpendicular angle relative to the base to accommodate particular cutting block designs and to place the block in a desired orientation relative to the tibia.
One aspect of the invention contemplates a method for calibrating a flexion gap relative to a known extension gap value as part of a knee arthroplasty procedure. This method can include the steps of placing the knee joint in flexion, distracting the knee joint, and then placing an instrument relative to the tibia that includes a scale juxtaposed to the distal end of the femur. The scale denotes a gap dimension between the prepared end of the tibia and a particular location on the prepared end of the femur. In the next step, a position on the scale is located that corresponds to a known gap value. All of these steps occur while maintaining the distraction of the knee joint.
In a preferred embodiment, the gap value corresponds to a pre-determined extension gap value. Thus, in a prior series of steps, the knee can be placed in extension, distracted using the distraction device, and the extension gap measured. This measurement can be located on the scale of the flexion gap instrument to determine a proper flexion gap location. This location on the scale can then be used to orienting a cutting block on the femur with a portion of the cutting block aligned with the located position on the scale corresponding to the known extension gap value. The inventive instrument is used to orient the cutting block parallel to the cut proximal end of the tibia while the ligaments are maintained in tension by a distraction tool. In this way, a flexion gap can be established that does not vary between the medial and lateral aspects.
The inventive method can include means for temporarily supporting the cutting block on the instrument. The cutting block can then be mounted on the femur to make the appropriate proximal and distal cuts of the distal end of the femur.
In certain aspect of the invention, the distraction device can be an independent knee balancer device. In other embodiments, the distraction device is a pivoting knee balancer. With each type of distraction device, the flexion gap instrument can be positioned or mated with the distraction plates of the instrument so as not to interfere with the operation of the distraction device.
In one embodiment of the invention, an instrument for establishing the flexion gap of a knee joint in flexion is provided that comprises a base portion configured for placement relative to the proximal end of the tibia with the knee joint in flexion. The instrument also includes a plate projecting from the base portion that is configured to be juxtaposed relative to the distal end of the femur when the base portion is placed relative to the tibia. In a preferred embodiment, the base is configured to rest directly on the tibial plateau, or within the tibial plateau mating plate of the distraction device or ligament balancing tool.
In one feature of the invention, indicia are carried by the plate to define a gap dimension measured from the proximal end of the tibia. In certain embodiments, at least a portion of the plate bearing said indicia is sized to be slidably received in a channel of a cutting block. The instrument can also include means for supporting the cutting block on said plate. In certain embodiments, this means for supporting can include a leaf spring connected to the plate, with the leaf spring arranged to bear against the channel of the cutting block.
In the preferred embodiment, the plate projects substantially perpendicularly from the base. In other embodiments, adapted for use with different cutting blocks, at least a portion of the plate bearing the indicia is oriented at a non-perpendicular angle relative to the base.
It is one object of the invention to provide an instrument for determining a flexion gap that can be used during a total knee arthroplasty procedure. A primary goal is to produce a prosthetic knee joint that maintains proper ligament tension in both extension and flexion.
One benefit of the present invention is that it provides a means for determining a proper flexion gap when the distal end of the femur is being prepared for the prosthesis. Another benefit is that the inventive instrument and method can be used concurrently with existing distraction devices, without interfering with the function of those devices.
Other objects and benefits of the invention will become apparent from the following written description taken with the accompanying figures.